Ducting allowance: I-joists vs floor trusses over my basement suite

Q: While studying the plans of my proposed new house project, I find that I may have to ‘drop’ the ceiling in my basement suite to allow for dryer and range venting which will be running perpendicular to the 2×12 engineered I-joist direction of the main floor. Floor truss joist (parallel chord floor truss) may be my solution to keep the ceiling up where it belongs and run ductwork inside the framework of the trusses. My maximum span is 17′. Will floor trusses work for this span? What OC will be required? How will the ‘bounce’ factor on the main floor be affected compared to regular engineered I-joist?


A: I agree with you in not dropping the ceiling height in your basement.  Engineered I-Joists are a great choice for floors as they can achieve much greater spans than conventional lumber and their dimensional stability means little or no shrinkage.  This all contributes to a well performing floor system.

Though I-Joists can be drilled or have holes cut in them to accommodate plumbing and ducting at select locations, if you’re not sure of where the ducting will be or you would like to place it near the outer edges of the span, then a 4×2 parallel floor truss system is the way to go.  A large “chase” can be incorporated into the design with little or no effect on deflection or performance.  This opening allows for larger air handling vents too.

The webs in a 4×2 truss are usually installed at roughly a 45 degree angle which allows for the greatest amount of room between the members.  With this design, you could literally run your vents and ducting within a foot of the bearing walls if need. (Or practically any location in between for that matter).

We can span 17’ at 12” deep and still maintain a 24” o/c spacing.  Keep in mind that you will need to use ¾” plywood if that is the spacing that you want.

Bounce?  Sorry but if you’re looking for a trampoline, we don’t sell those!  All kidding aside, bounce or “deflection” is determined by several factors when designing a 4×2 floor truss.  BC Building Code requires a floor to perform at L/360 meaning that you would take “L” in inches and divide by 360.  This would give you the maximum deflection allowed in your floor system by code.  A 17’ span would work out to (17×12)/360 = 0.56 inches.  We try to design our I-Joist floors at L/480 which would limit your deflection to (17×12)/480 = 0.42 inches.  A 4×2 floor truss can easily exhibit numbers better than L/1000 or 0.20 inches.

One more thing to consider in design is vibration.  Long, slender floor members tend to vibrate under certain conditions and the I-Joist or floor truss design will address this factor.  I-Joists allow for bridging and strapping between them and 4×2 floor trusses allow for continuous “strongbacks” to be attached to the vertical members.  This can really strengthen up your whole floor design.

So, the choice is up to you.  I hope that I have addressed your questions but if not, please feel free to contact me directly should you have any more concerns.

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